Polyisoprene is a key material for producing a broad range of consumer and industrial products. The two most common forms for polyisoprene are “natural rubber” and “synthetic polyisoprene”. Natural rubber typically is derived from latex produced by Hevea brasiliensis (i.e., the common rubber tree), although a broad range of other plants (e.g., guayule and Taraxacum kok-Saghyz (aka Russian dandelion)) also are known to produce stoichiometrically similar, rubber-like materials. Unlike natural rubber, which is only formally derived from polymerization of isoprene, synthetic polyisoprene is actually produced by large-scale, industrial polymerization of isoprene monomer.
The structures of synthetic polyisoprene (PI) and natural rubber (NR) are similar enough to allow for free substitution of either rubber in many applications, but there are important differences. For example, rubber produced by the rubber tree has a high molecular weight and a tendency to crystallize more completely and faster than commercially available synthetic PI. The high molecular weight is desirable for imparting “green strength” during tire manufacturing. The rapid strain-crystallization of rubber is believed to be responsible for the excellent wear and tear properties of natural rubber—especially under severe conditions.
Early efforts to develop synthetic PI as a replacement for natural rubber elucidated much of the fundamental technology and allowed commercialization of synthetic PI to be achieved in the 1960's. (see e.g. Schoenberg, et al Rubber Chem. Tech. 52, 526-604 (1979)) In general, the following characteristics are believed to be desirable in synthetic PI intended for tire applications: high cis-content (vs trans content); high 1,4-addition (vs 3,4-addition); high head-to-tail content; and high molecular weight.
Subsequent efforts to achieve the highest practical level for each characteristic—especially using Neodymium-based Ziegler/Natta-type catalysts have built upon the early work and led to today's best synthetic replacements for NR. (see e.g. Friebe, et al Adv. Polym. Sci. 204, 1 (2006))
For several decades, it was believed that the differences between natural rubber and synthetic rubber were the result of natural rubber having an almost pure cis-1,4 stereochemistry and branched polymer chain structures. The potential role of non-rubber constituents in natural rubber was largely ignored. It now appears from extensive recent work by Prof. Yasuyuki Tanaka and coworkers that the non-rubber components play an essential role in determining the properties and performance of natural rubber. (see e.g., Tanaka, et al Polymer 41, 7483-8 (2000); Rubber Chem. Tech. 74, 355-75 (2001); Biopolymers 2, 1-25 (2001)) This is particularly true for Hevea rubber, which clearly has a structure with nanometer-scale phase domains that can explain many of the property differences between natural rubber and synthetic rubber. In other words, natural rubber is best viewed as a nanostructured elastomer rather than a hydrocarbon polymer with non-hydrocarbon impurities.